Various technologies exist for detecting residues of certain substances of interest, such as explosives and illicit drugs. Some trace detection technologies use spectrometric analyses of ions formed by the ionization of vaporized substances of interest. Spectrometric analyses include ion mobility spectrometry and mass spectrometry, for example, both of which are common in trace detection.
A general requirement for the trace detection of explosives by mass spectrometry and ion mobility spectrometry is the transfer of surface deposited explosives into a gas phase as neutrals or ions, independent of the morphology and/or chemistry of both the explosives and the surface. Currently available methods operating at atmospheric pressure consist of either evaporating the analyte by thermal energy from a surface, followed by an ionization step using ions generated by a bipolar ion source (a low temperature plasma) or unipolar ion source (created by, for instance, an electrospray source) before ion mobility or mass spectrometry analysis, or surface disruption by dissolving the analyte followed by an electric-field-induced surface lift-off of ionic components created or deposited on the surface. The thermal evaporation step has limited compatibility with thermally sensitive surfaces and requires either a radioactive source of ions or an AC or DC plasma source of ions. The disruption by surface solvation requires the additional modification of instrumentation and suffers from low sensitivity, while electrode erosion and ozone generation may be caused by AC and DC plasma sources.
Conventionally, a surface may be swabbed using material such as polytetrafluoroethylene coated fiberglass to physically separate residue from a surface for analysis. The material containing the surface residue, also referred to as a trap, is then heated to vaporize the residue into a gas phase. For example, the trap may be introduced to an inlet of a desorber, where the trap is rapidly heated to a desired temperature by a heating element to vaporize the residue. Because it is necessary to ionize the vaporized residue before mobility or mass spectrometry analyses can take place, the vaporized residue is transported from a vaporization region to a separate ionization region in which ionization is performed using ions generated by a bipolar ion source or a unipolar ion source. The resulting ionized gas phase is then transferred to an analysis device or a detector for spectrometric analysis where it is screened for the substance of interest.
U.S. Pat. No. 8,742,363, titled ‘Method and apparatus for ionizing gases using UV radiation and electrons and identifying said gases’, discloses “a method for ionizing and identifying gases, wherein the gases to be identified are ionized in a reaction chamber and the product ions are measured, wherein the measurement of the product ions takes place via electrical fields acting on the product ions and the detection is performed with a detector for ions. It is provided that ionization takes place via UV radiation, and that simultaneously or sequentially ionization by electrons takes place.”
U.S. Pat. No. 5,968,837, titled ‘Photo-ionization ion mobility spectrometry’, discloses “a method for photo-ionization ion mobility spectrometry, a reagent gas is added, in particular an aromatic compound, which has a large ionization cross section in the range of ionizing VUV radiation, but a low probability for the formation of protonated quasi-molecular ions. In this way, the detection of only weakly proton affine substances is also amplified or even made possible at all, and also the detection of electronegative substances in a negative operating mode is improved.”
U.S. Pat. No. 5,969,349, titled ‘Ion mobility spectrometer’, discloses an “ion mobility spectrometer with a non-radioactive electron source to generate ions inside a reaction chamber. The reaction chamber consists of two partial chambers, one of which is evacuated and comprises the electron source, and the other one is connected to the drift chamber of the IMS via a shutter grid. The partition wall between both partial chambers is transparent to electrons but impermeable for gas molecules. The electron source may comprise a thermoemitter or a photocathode, which is illuminated from outside through a window.”
U.S. Pat. No. 6,586,729, titled ‘Ion mobility spectrometer with non-radioactive ion source’, discloses “an ion mobility spectrometer (IMS) that has a non-radioactive electron source in an evacuated chamber that is separated from the reaction chamber of the IMS by an x-ray window. Electrons from the source impinge upon an x-ray anode, causing the emission of x-ray radiation toward the window. A current controller is provided by which currents in the electron-source chamber are monitored and controlled using a microprocessor circuit. If a maximum permissible residual gas pressure is exceeded, the electron source is automatically shut down and a gettering process is activated.”
U.S. Pat. No. 6,429,426, titled ‘Ionization chamber with electron source’, discloses an “ionization chamber, especially for an ion mobility spectrometer, with a non-radioactive electron source. The chamber consists of two compartments, of which one is evacuated and contains an electron source, and the other represents the reaction chamber of the IMS. In the evacuated compartment, X-ray quanta are produced in an anode by electron bombardment and these X-ray quanta can penetrate a partition between the two compartments. The partition between the two compartments is impermeable to electrons from the source and to gas molecules. In one or several conversion layers within the reaction compartment, X-ray quanta are converted to quanta of a lower energy and/or photoelectrons that can ionize the air constituents at a high level of efficiency.”
United States Patent Application No. 2002/0185594, titled ‘Ion mobility spectrometer with mechanically stabilized vacuum-tight x-ray window’, discloses that “an ion mobility spectrometer (IMS) has a non-radioactive electron source and an x-ray anode in an evacuated chamber. The impinging of electrons from the source on the anode results in the generation of x-ray radiation. The x-ray radiation passes through a window that provides a vacuum barrier between the electron source chamber and a reaction chamber of the IMS by an x-ray window. A support grid is attached to the reaction-chamber side of the x-ray window, and mechanically stabilizes the window.”
Such conventional trace detection systems that require the use of a trap or other means for obtaining a sample have significant drawbacks including limitations in throughput and scalability. The physical act of removing surface material requires a process separate from the actual analysis, and the throughput of sample heating, such as in a desorber, may be limited due to the time needed to cool the desorber to accept a subsequent sample.
It is realized herein that the use of a trap and separate desorber may limit the performance and scalability of a trace detection system. The downtime between heat up and cool down of a desorber limits its throughput. Additionally, the required heating and cooling system may be excessively bulky and unable to be downscaled, requiring a fixed system that is not easily transported or handled in the field. In addition, the trace detection system requires consumable goods such as a trap for removal of surface residues from an object being tested.
Hence, there is need for a trace detection system that does not require a desorber and a separate ionization step. There is also need for a trace detection system that can be used to detect a wide range of substrate materials, can easily be miniaturized, has a rapid cycle time, and relatively low power requirements. There is also need for a trace detection system that does not cause ozone formation and has a small footprint.